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Artifacts in the electron paramagnetic resonance spectra of C60 fullerene ions: Inevitable C120O impurity

  • Author(s): Paul, P
  • Kim, KC
  • Sun, D
  • Boyd, PDW
  • Reed, CA
  • et al.

Published Web Location

https://doi.org/10.1021/ja011832f
Abstract

Aspects of the electron paramagnetic resonance (EPR) spectra of C60n- fulleride ions (n = 2, 3) and the EPR signal observed in solid C60 are reinterpreted. Insufficient levels of reduction and the unrecognized presence of C120O, a ubiquitous and unavoidable impurity in air-exposed C60, have compromised most previously reported spectra of fullerides. Central narrow line width signals ("spikes") are ascribed to C120On- (n = odd). Signals arising from axial triplets (g ∼ 2.0015, D = 26-29 G) in the spectrum of C602- are ascribed to C120On- (n = 2 or 4). Their D values are more realistic for C120O than C60. Less distinct signals from "powder" triplets (D ∼ 11 G) are ascribed to aggregates of C120On- (n = odd) arising from freezing nonglassing solvents. In highly purified samples of C60, we find no evidence for a broad ∼30 G signal previously assigned to a thermally accessible triplet of C602-. The C602- ion is EPR-silent. Signals previously ascribed to a quartet state of the C603- ion are ascribed to C120O4-. Uncomplicated, authentic spectra of C60- and C603- become available when fully reduced samples are prepared under strictly anaerobic conditions from freshly HPLC-purified C60. Solid off-the-shelf C60 has an EPR signal (g∼ 2.0025, AHpp ∼ 1.5 G) that is commonly ascribed to the radical cation C60.+. This signal can be reproduced by exposing highly purified, EPR-silent C60 to oxygen in the dark. Doping C60 with an authentic C60.+6 salt gives a signal with much greater line width (AHpp = 6-8 G). It is suggested that the EPR signal in air-exposed samples of C60 arises from a peroxide-bridged diradical, •C60-O-O-C60. or its decomposition products rather than from C60.+. Solid-state C60 is more sensitive to oxygen than previously appreciated such that contamination with C120O is almost impossible to avoid.

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